1. Field of the Invention
This invention relates to a hand operated pump for use in calibration of pneumatic instruments.
2. Brief Description of the Prior Art
The usefulness of pressure calibration pumps is generally defined in terms of (1) the maximum pressure which may be established within the trapped volume of the calibration interconnection system, (2) the precision of pressure adjustment which may be set, particularly at high pressures, and (3) the fineness of bleed adjustment which can be made over a large number of use cycles.
The maximum pressure which can be achieved by any pump is fixed by its compression ratio, this being the quotient of maximum pump volume to minimum pump volume. Larger numbers are better and are achieved by making the volume represented by the intake and exhaust valves as small as possible.
The typical arrangement for calibrating the pressure applied to a device under test (DUT) includes the pressure sensitive instrument to be calibrated or device under test (DUT), a precision pressure gauge (calibrator or reference standard) and a hand pump with volume adjustment or volume trim and bleed or relief valve with interconnecting tubing and fittings. The prior art pump, as shown in FIG. 2, generally comprises a first cylinder 13 with a first piston 15, a second cylinder 17 with a second piston 19 and a relief valve 9. The volume of the second cylinder 17 may be changed by rotating the volume trim knob 7, causing the second piston 19 to move within the second cylinder, changing the volume of the second cylinder as well as the total trapped fluid volume. By this means, the captured pressure may be adjusted to the precisely required value.
The conventional precision pressure adjustment is made by moving the second piston 19 within the second cylinder 17 by means of screw threads. The force acting on the screw threads is equal to the internal pump pressure multiplied by the projected area of the second piston 19, less the outside or atmospheric pressure acting on the projected area of the backside of the second piston. The net pressure induced force increases with internal pressure. That force multiplied by the coefficient of friction of the screw threads opposes the operator attempted movement of the threads. Eventually a pressure is reached at which the turning of the adjustment knob 7 becomes inordinately difficult and precise volume adjustment is no longer possible.
Bleed or relief valves of prior art design, which are generally used with the second cylinder/piston device 17, 19, employ conically shaped needles within matching conical valve seats. Typically, a screw arrangement moves the needle within its seat. The gap between the needle and its seat fixes the flow rate of the escaping fluid, that is the rate at which the internal compressed air escapes or bleeds to the atmosphere. To provide complete sealing, needle and seat must be perfectly matched and be jammed together to less than gas molecule dimensions. This jamming together of needle and seat, aided by dirt in the air, eventually causes damage to the mating parts and the valve commences leaking. Choice of needle and seat materials and addition of special end of stroke sealing provisions can overcome the eventual leaking problem, but always at the expense of fineness of adjustment. The traditional design is also subject to damage by overtightening.